With the development of electronics and computer industries, precise polishing of metallic materials such as substrate materials for magnetic disks, e.g., aluminum, a nickel-based alloy, etc., and substrate materials for semiconductors, e.g., silicon, gallium-arsenic, etc., has become a very important technique.
Recently, in magnetic disks and semiconductors, a recording density and an integration degree are increased, and in a precise polishing work of these substrate materials, the levels required for the surface smoothness and no defects (i.e., no scratches, orange peels, pits, nodules) on the polished surfaces have been more and more advancing. Accordingly, the improvement of the level of the precise polishing technique has become more important than ever.
Hitherto, a lapping work or a polishing work using free polishing agents (abrasives) has been practiced from old as a working method of obtaining a precision on a polished surface. Powders of alumina, silica, cerium oxide, zirconium oxide, iron oxide, chromium oxide, silicon carbide, diamond, etc., have been used as the polishing agents.
In these polishing agents, since alumina has a high hardness and an excellent polishing efficiency, and can be easily available, alumina has been widely utilized as polishing agents for metallic materials such as substrate materials for magnetic disks and semiconductors, etc.
From the view point of improving the surface smoothness and the removal rate on the polishing surface in the precise polishing work for substrate materials for magnetic disks and semiconductors, a specific polishing accelerator compound with water and an alumina polishing agent have been proposed.
For example, U.S. Pat. No. 4,696,697 discloses a neutral or weak acidic polishing composition for plastic articles and memory hard disks using nickel sulfate as a polishing accelerator, and U.S. Pat. Nos. 4,075,566 and 4,769,046 discloses a polishing composition for memory hard disks containing a polishing accelerator such as nickel nitrate and aluminum nitrate.
However, in the case of using nickel sulfate or nickel nitrate as a polishing accelerator, the pH is neutral or weakly acidic but the polishing composition has disadvantages that the polishing efficiency is insufficient and surface defects such as fine pits and nodules are liable to occur.
In the case of using aluminum nitrate as a polishing accelerator, the polishing composition shows a high polishing efficiency as compared with the case of using nickel sulfate or nickel nitrate but since the pH thereof is strongly acidic, of from 2 to 4, the polishing composition has disadvantages that the working property is poor and the polishing apparatus is corroded.
Furthermore, the surface smoothness of the polished surface can not attain the required level even by using the foregoing polishing accelerators, and hence a polishing composition having a high polishing efficiency and giving a polished surface having a good surface smoothness has been desired.
Under such circumstances, as the result of various investigations for obtaining a polishing composition for metallic materials having a high polishing efficiency and giving polished surfaces having less surface roughness and having no surface defects, the present inventors have discovered that when a specific polishing accelerator is used, a polishing composition for metallic materials satisfying all the requirements described above can be obtained and have succeeded in accomplishing the present invention.